First off, congrats flightpattern, you're in a great position.
I'm BA P2 LH and reasonably senior. I agree with your pro's and con's although I'd also highlight that the time to command in Virgin is likely to be very much shorter than in BA. The current time to command for LH in BA is around 18+ years, whereas my mates in Virgin seem to be coming up for command after about 12 years at the moment. From what they tell me that time to command is likely to reduce further due to the upcoming retirement bulge in Virgin. While BA recruitment is racing ahead we still have a lot of relatively young LH Captains as a product of the cadet scheme back in the 90's who understandably wont be going anywhere for a while yet. BA short haul commands are currently coming up after as little as 2 or 3 years although there's a reason for this! I think you'd struggle to find many pilots in BA who would say that full time BA SH is a sustainable career choice so I'd weigh that against the pro of being able to switch between LH and SH.
My own experience is that most BA LH pilots will be doing something north of 800 hours before overtime. As stated above blindlines, reserve and or broken aircraft ala 787, will keep your hours lower than average although pilot shortages on certain fleets and triplines will keep your hours high.
The rotating seniority at Virgin is a massive plus to my mind. I did nearly 10 years of blind lines albeit with hindsight I could and should have bid off my fleet sooner to get more control. My situation was unusual as a result of the retirement age being extended and being on a very senior fleet but my point is that if there is unforeseen stagnation for any reason you could find yourself stuck at the bottom for a long time whereas at Virgin at least you'd still have the opportunity to have some control over your working life. Just note that on achieving a new command at BA, you'll be back to the bottom of your seniority list and have to work your way up all over again, although you would go across to the P1 pay scales at your current pay point whereas in Virgin you'd go back to P1 pp1.
The pensions at both airlines are similar now but bear in mind you'd be joining BA onto 34 paypoints (changed from 24 a few years ago) also that Virgin has a higher starting salary and the potential for a much quicker command at Virgin while working approx 10% fewer hours than at BA and I could certainly see a strong case for going to Virgin over BA. I have a mate who joined BA in 2004 and shortly thereafter resigned to go to Virgin. When he resigned the BA manager was aghast as to why he would leave BA to join a lesser airline! He's now a Captain, quite happy as far as I can tell and steadily moving up the seniority list whereas in BA he'd be a relatively senior FO on any LH fleet, with about 4-5 years to go to command, earning a lot less than he currently does and working 10% harder for it!
Good luck with whatever you decide.